1. Field of the Invention
The present invention relates to an electrochemical cell and a process for converting anhydrous hydrogen halide to halogen gas using a membrane-electrode assembly or a separate membrane and electrodes, such as gas diffusion electrodes.
2. Description of the Related Art
Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinyl chloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product.
A number of commercial processes have been developed to convert HCl into usable chlorine gas. See, e.g., F. R. Minz, "HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, Apr. 21-23, 1993. The commercial processes fall into two categories: thermal catalytic oxidation processes and electrochemical processes.
The current commercial thermal catalytic oxidation processes for converting anhydrous HCl and aqueous HCl into chlorine are the "Shell-Chlor", the "Kel-Chlor" and the MT-Chlor" processes. These processes are based on the Deacon reaction. Another thermal catalytic oxidation process based on the Deacon reaction which is currently being investigated, but which is not yet commercial, is the Minet process. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as a catalyst. The commercial processes based on the Deacon reaction have used other catalysts in addition to or in place of the copper used in the original Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide and chromium oxide in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on processing equipment produced by harsh chemical reaction conditions associated with these processes. However, in general these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 200.degree. C. and above.
Electrochemical processes convert aqueous HCl to chlorine gas by passing direct electrical current through a solution. The current commercial electrochemical process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22% is fed at 65.degree. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17% with the production of chlorine gas and hydrogen gas. The chlorine gas produced by the Uhde process is wet, usually containing about 1% to 2% water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps./ft..sup.2 (5.38 kA/m.sup.2), because of this side reaction. The side reaction results in reduced electrical efficiency and corrosion of the cell components.
Furthermore, electrolytic processing of aqueous HCl can be mass-transfer limited. Mass-transfer of species is very much influenced by the concentration of the species as well as the rate of diffusion. The diffusion coefficient and the concentration of species to be transported are important factors which affect the rate of mass transport. In an aqueous solution, the diffusion coefficient of a species is .about.10.sup.-5 cm.sup.2 /sec. In a gas, the diffusion coefficient is dramatically higher, with values .about.10.sup.-2 cm.sup.2 /sec. In normal industrial practice for electrolyzing aqueous hydrogen chloride, the practical concentration of hydrogen chloride or chloride ion is .about.17% to 22%, whereas the concentration of hydrogen chloride is 100% in a gas of anhydrous hydrogen chloride. Above 22%, conductance drops, and the penalty, in terms of additional power, for electrolyzing hydrogen chloride begins to climb. Below 17%, oxygen can be evolved from water, corroding the cell components, reducing electrical efficiency, and contaminating the chlorine.
U.S. Pat. No. 4,311,568 to Balko also describes an aqueous electrochemical process for converting HCl to chlorine. However, aqueous electrochemical processes for converting HCl to chlorine are hampered by oxygen evolution. Oxygen evolution occurs when there is chloride starvation in the anode, and the cell current is sustained by the electrolysis of water derived from aqueous hydrogen chloride and/or from water within a hydrated membrane. Thus, in Balko, controlling and minimizing oxygen evolution is an important consideration. In general, the rate of an electrochemical process is characterized by its current density. In Balko, as overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities for a short period of time, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at higher current densities for any length of time, the anode would be destroyed.
Some electrochemical cells, such as that disclosed in U.S. Pat. No. 4,311,568 to Balko, employ a membrane and electrodes which are physically separate elements. Such an arrangement has non-uniformities in both the membrane and the electrodes, resulting in uneven contact therebetween and less utilization of the catalyst than if the contact between the membrane and the electrodes were uniform. Accordingly, the current density of such a cell is limited not only by the presence of water, as discussed above, but also by catalyst utilization. Improved catalyst utilization has been achieved by a membrane-electrode assembly, as disclosed in U.S. Pat. No. 5,330,860 to Grot and Banerjee. This Patent discloses the use of a membrane-electrode assembly in an aqueous electrolytic cell or a fuel cell.
Thus, there exists a need to develop an electrochemical cell which is able to directly convert anhydrous hydrogen halide to essentially dry halogen gas which can achieve much higher current densities than can be achieved by electrochemical cells of the prior art. In addition, there exists a need to develop an electrode system which has improved catalyst utilization than electrodes of the prior art.